German Cancer Research Center (DKFZ), Department of Medical Physics in Radiation Oncology, Im Neuenheimer Feld 280, D-69120 Heidelberg, Germany.
Phys Med Biol. 2010 Jul 7;55(13):3741-51. doi: 10.1088/0031-9155/55/13/011. Epub 2010 Jun 9.
For quality assurance measurements in radiotherapy with scanned ion beams, such as beam profile measurements and dosimetrical verification of patient plans, high spatial accuracy is required. We investigated the Gafchromic EBT film, which is the ancestor of the currently available EBT2 film, for this purpose. The excellent spatial resolution and improvements in sensitivity as compared to older types of radiochromic films make it a suitable candidate. In this contribution a study of the dose-response relation for monoenergetic beams (plateau), based on the absolute dose, is presented. We found the film response to be comparable for protons and photons. For carbon ions we observed a decrease in the darkening efficiency of about 30% with respect to photons. This effect is likely to be caused by the ion track structure. We did not find any dependence of the relative efficiency on the applied dose and energy over typical ranges used in ion beam therapy. Furthermore, it is shown that overlaps of ion tracks are not an issue for EBT film dosimetry in medical ion beams. The above findings enable the use of EBT films calibrated in a beam of a particular ion type for dose measurements at any other dose and energy. The uncertainty of the film darkening determination in ion beams is also discussed.
为了确保扫描离子束放射治疗的质量，如束流剖面测量和患者计划的剂量验证，需要很高的空间精度。为此，我们研究了 Gafchromic EBT 薄膜，它是目前可用的 EBT2 薄膜的前身。与旧类型的放射色胶片相比，其具有出色的空间分辨率和更高的灵敏度，是一种合适的候选胶片。在本研究中，我们对单能束（平台）的剂量响应关系进行了研究，该研究基于绝对剂量。我们发现，胶片对质子和光子的响应相当。对于碳离子，我们观察到与光子相比，黑化效率降低了约 30%。这种效应可能是由离子径迹结构引起的。我们没有发现相对效率在离子束治疗中使用的典型剂量和能量范围内与所施加剂量和能量有关。此外，还表明在医学离子束中，EBT 胶片剂量测定中离子径迹的重叠不是问题。上述发现使得可以使用在特定离子类型的射束中校准的 EBT 胶片，在任何其他剂量和能量下进行剂量测量。还讨论了离子束中胶片黑化测定的不确定度。
